1. Field of the Invention
The present invention relates to a method and apparatus for monitoring and controlling toner image formation processes, and in particular, toner image formation processes in xerographic or ionographic type printing machines which utilize transfer members for transferring a toner image from an imaging member to a receiving material, such as paper.
2. Description of Related Art
A typical xerographic printing machine (such as a photocopier, laser printer, facsimile machine or the like) employs an imaging member (e.g., a photoreceptor) that is uniformly electrostatically charged, and then exposed to a light image corresponding to an image to be printed so that the imaging member is selectively discharged in accordance with the image. Thus, exposure of the imaging member records an electrostatic (latent) image on it corresponding to the informational areas contained within the image to be printed. This latent image is developed by bringing a developer material (liquid or powder) into contact with the latent image to form a toner image. The toner image recorded on the imaging member is then transferred to a receiving material such as paper either directly or via an intermediate transport member. The receiving material can be transported past the imaging member to receive the toner image by a transported past the imaging member to receive the toner image by a transport member such as, for example, an electrostatic belt or drum which presses the receiving material against the toner image on the imaging member to receive the toner image therefrom. Alternatively, the toner image can be transferred from the imaging member to an intermediate belt or drum which then further transfers the toner image to the receiving material. The developed toner image on the receiving material usually is then subjected to heat and/or pressure to permanently fuse the image to the receiving material.
In an ionographic printing machine, a latent image corresponding to an image to be printed is formed on an imaging member by an ion generating device which selectively propels ions toward the imaging member in accordance with an input signal to form the latent image on the imaging member. Development of the latent image into a toner image, and transfer of the toner image to a receiving material can proceed in a manner similar to that described above with respect to the xerographic printing machine.
With the increase in use and flexibility of printing machines, especially color printing machines which print with two or more different colored toners, it has become increasingly important to monitor the toner development process so that increased print quality, stability and control requirements can be met and maintained. For example, it is very important for each component color of a multi-color image to be stably formed at the correct toner density because any deviation from the correct toner density may be visible in the final composite image. Additionally, deviations from desired toner densities may also cause visible defects in mono-color images, particularly when such images are half-tone images.
It is known to monitor the developed mass per area (DMA) for a development process by using densitometers such as, for example, infrared densitometers (IRDs) to measure the mass of a toner patch formed on the imaging member. Although DMA permits one to monitor the manner in which toner is deposited onto the imaging member, it does not provide a complete picture of the image formation process because DMA does not take into account the efficiency with which toner is transferred from the imaging member to the receiving material (e.g., paper) which ultimately receives the final image. Conventional printing machines do not attempt to measure the toner transfer process (i.e., the process by which the toner image is transferred from the imaging member to the receiving material (paper sheet)) because transferring a control toner patch to a sheet of paper would necessitate the disposal of the sheet which cannot be done on a routine basis.
Another example of a process control which is monitored includes the developability. Developability is the rate at which development (toner mass/area) takes place. Developability is typically monitored (and thereby controlled) using densitometers (e.g., IRDs) and by measuring toner concentration (TC) in the developer housing. As described above, IRDs measure total developed mass (i.e., on the imaging member), which is a function of developability and electrostatics. Thus, the developability cannot be determined using IRDs alone because the electrostatics of the imaging member also affect the mass of toner deposited on the imaging member by a developer device. TC is measured by directly measuring the percentage of toner in the developer housing (which, as is well known, contains toner and carrier particles). However, the relationship between TC and developability is affected by other variables such as ambient temperature, humidity and the age of the toner. For example, a 3% TC results in different developabilities depending on the variables listed above. Thus, maintaining TC at a predetermined value does not ensure a desired developability.
Accordingly, the monitoring and control of the toner development process can be difficult and complicated. It is desirable to provide new and useful means for monitoring the toner development process so that the toner development process can be appropriately controlled.